Histamine antagonists have been studied for many years. In the last forty years, various substances, such as mepyramine, were found to antagonize certain effects (H.sub.1) of histamine. These substances however had no effect on gastric acid secretion which is mediated by other histamine receptors (H.sub.2) [Black et al., Nature 236, 385, (1972)]. With the advent of cimetidine, preceded by two other compounds (burimamide and metiamide), a new type of antihistamines (H.sub.2 -blockers) has been discovered which are capable of antagonizing those responses (for instance gastric acid secretion) which are unaffected by classical antihistamines (H.sub.1 -blockers). More recently, new H.sub.2 -receptor antagonists, that is, ranitidine [Bradshaw et al., Brit. J. Pharmacol. 66, 464 P, (1979)], tiotidine [P. O. Jellin, Life Sci., 25, 2001, (1979)] and BL 6341 [Cavanagh et al., Fed. Proc., 40, 2652, (1981)], have been reported to be more potent than cimetidine, both as H.sub.2 -receptor antagonists "in vitro" and as inhibitors of gastric acid secretion "in vivo".
All of these compounds share a common feature, namely a methylthioethyl side-chain (--CH.sub.2 SCH.sub.2 CH.sub.2 --) bearing neutral polar groups connecting basic or basic-substituted heterocyclic rings.
In our Italian Patent Application Ser. No. 26323 A/80 we have described a new type of H.sub.2 -receptor antagonists, that is, imidazolyl-phenyl-amidines, which are potent H.sub.2 -receptor antagonists and inhibitors of gastric acid secretion. With respect to cimetidine, the imidazole ring is retained in these compounds, whereas the methyl-thioethyl side-chain is replaced by a phenyl ring connected to an amidino group.